Conventionally, a flash memory has typically been used as a nonvolatile semiconductor storage device.
In this flash memory, as shown in FIG. 28, a floating gate 902, an insulation film 907 and a word line (control gate) 903 are formed in this order via a gate insulation film 908 on a semiconductor substrate 901, and a source line 904 and a bit line 905 are formed on both sides of the floating gate 902, constituting a memory cell. Around this memory cell are formed element isolation regions 906 (refer to Japanese Patent Laid-Open Publication No. HEI 5-304277).
The memory cell retains storage as the quantity of charge in the floating gate 902. In the memory cell array constructed by arranging the memory cells, the desired memory cell can be subjected to rewrite and read operations by selecting the specified word line and bit line and applying a prescribed voltage to the lines.
The flash memory as described above exhibits a drain current Id to gate voltage Vg characteristic indicated by the solid line curve and the dashed line curve in FIG. 29 when the quantity of charges in the floating gate 902 changes. That is, if the quantity of negative charges in the floating gate 902 is increased, then the characteristic curve changes from the characteristic indicated by the solid line curve to the characteristic indicated by the broken line curve in FIG. 29, and the Id-Vg curve is displaced roughly parallel in a direction in which the gate voltage Vg increases with respect to same drain current Id, and the threshold voltage increases.
However, the flash memory as described above has been functionally required to arrange the insulation film 907 that isolates the floating gate 902 from the word line 903 and had difficulties in reducing the thickness of the gate insulation film to prevent the leak of charges from the floating gate 902. Therefore, it has been difficult to effectively reduce the thickness of the insulation film 907 and the gate insulation film, and this has hindered the miniaturization of the memory cell.
Therefore, the semiconductor storage device provided with the memory cell array that employs the aforementioned memory elements as memory cells has the problem that reliable operation cannot be assured with a further reduced circuit area.
Moreover, the semiconductor storage device that employs the aforementioned memory elements as memory cells in the memory cell array can also be sufficiently supplied with a current when an input voltage is supplied directly from the outside. However, it is sometimes the case where the input voltage largely fluctuates to fall below the desired voltage. Therefore, in order to supply an accurate voltage to the memory cell array, a charge pump is needed. However, there is a problem that the current tends to become insufficient via the charge pump, failing in achieving stable operation and leading to degraded reliability.